Field of the Invention
The present invention relates to methods of correcting magnification in the main scanning direction in image forming apparatuses, such as multi-function printers (MFPs) and the like.
Description of the Related Art
A conventional image forming apparatuses includes an optical scanning device that emits a laser beam for scanning a photosensitive member. The optical scanning device includes a laser light source that emits a laser beam and a deflection unit (e.g., rotatable polygon mirror) that deflects the laser beam emitted by the laser light source so as to scan the photosensitive member with the laser beam. The laser beam deflected by the deflection unit is imaged on the photosensitive member through an optical lens, such as an f-theta (fθ) lens, designed such that the scanning speed on the photosensitive member is substantially constant.
Although the optical lens is designed with high precision, it is not possible to fabricate an optical lens with precision that satisfies the precision of the design, and thus there is often a small error in the optical performance of the optical lens. Therefore, if that error is not corrected, the magnification of an image in each of a plurality of regions in the main scanning direction may vary.
In order to address such errors, Japanese Patent Laid-Open No. 2013-22913 discloses an image forming apparatus that corrects driving data for driving a laser light source in units of smaller than one pixel. With the image forming apparatus disclosed in Japanese Patent Laid-Open No. 2013-22913, the magnification of an image in the main scanning direction may be obtained without errors.
In the meantime, there is an increasing demand for cost reduction of an optical scanning device in order to further reduce the cost of an image forming apparatus. One way to achieve cost reduction of an optical scanning device is to design an optical scanning device that is not provided with a scanning lens or to design an optical scanning device provided with a less expensive scanning lens that is inferior to a conventional optical lens in terms of the optical performance.
However, an optical scanning device that is not provided with a scanning lens or an image forming apparatus that is provided with an inexpensive scanning lens may not satisfy the requirements for high-quality imaging.
FIG. 13 illustrates a variation of the image magnification in the main scanning direction (variation in the scan width per unit time) in an optical scanning device that is not provided with a scanning lens. The main scanning direction corresponds to the direction in which a laser beam scans a photosensitive member. In FIG. 13, the horizontal axis represents the position in the main scanning direction (main scanning position/image height). The vertical axis in FIG. 13 represents the magnification of the image.
In the optical scanning device that is not provided with a scanning lens, the scanning speed of the laser beam is higher toward an end portion of the photosensitive member. Therefore, when a laser light source is driven in a condition in which the number of pieces of bit data per pixel in each region is set to be constant and the bit data is output to a laser driver in accordance with a clock signal of a constant cycle from a controller, the magnification of the image increases toward the end portion of the photosensitive member, as illustrated in FIG. 13. For example, in some image forming apparatuses, the correction magnification at positions that are ±150 mm from the respective end portions of the photosensitive member is as high as 30%, as illustrated in FIG. 13. The width of the image at each scanning position can be made substantially uniform by electrically correcting the image width by a reciprocal of the image magnification illustrated in FIG. 13.
Digital pulse width modulation (PWM) is a known method for correcting scan unevenness such as pixel magnification at positions away from the respective end portions of the photosensitive member. In the digital PWM correction method, a bit pattern containing a plurality of pieces of bit data is generated, for example, by changing the number of bits in a bit pattern, which is multi-valued image data indicating the density of an image. The bit data contained in the generated bit pattern is binary data that turns ON or OFF the laser light source. The image forming apparatus outputs the bit data contained in the bit pattern, one bit by one bit, in synchronization with an image clock of a constant frequency, and controls ON/OFF states of the laser light source. Processing for correcting the magnification of an image by changing the number of pieces of bit data contained in a bit pattern in accordance with the scanning position in the main scanning direction is performed according to the digital PWM.
For example, an image forming apparatus that converts image data corresponding to one pixel to a bit pattern that contains 24 pieces (N=24) of bit data and that corrects the number of pieces of bit data in this bit pattern will be considered. This image forming apparatus corrects the magnification M of each pixel in the main scanning direction in a range of 1≦M≦1.33. The correction of magnification for each pixel is defined by (N+D)/N=M. Here, N is the number of pieces of bit data corresponding to one pixel, D represents the number of pieces of bit data to be added to the bit pattern of the one pixel, and M is the magnification of the one pixel as function of its position along the main scanning direction. D is set to any one of the integers in a range of 0≦D≦8 in accordance with the position of the pixel in the main scanning direction. Desired magnification correction can be achieved by varying D for each pixel. For example, when D is 0, the magnification turns out to be (24+0)/24=1. When D is 8, the magnification turns out to be (24+8)/24=1.33. Accordingly, the data that indicates the correction magnification for each pixel may contain the bits in a number that can represent the number of integers that D can take. In the foregoing example, the number of integers that D can take is nine ranging from 0 through 8, and thus the data set can be expressed in four (≧log29, and integer) bits.
As illustrated in FIG. 13, for an image forming apparatus including an optical scanning device that is not provided with a scanning lens, the correction magnification needs to be determined in accordance with the pixel position in the main scanning direction (horizontal axis in FIG. 13). However, moire pattern may be generated when similar magnification correction is carried out on each pixel in a plurality of lines.
FIGS. 14A-1 through 14B-2 illustrate examples in which moire is produced. Each block in FIG. 14A-1 corresponds to one pixel, and each pixel is constituted by a bit pattern represented by a 24-bit data set. FIG. 14A-2 illustrates an image formed when a PWM signal is generated through the bit patterns illustrated in FIG. 14A-1 and a laser light source is driven by this PWM signal. Meanwhile, each block in FIG. 14B-1 corresponds to one pixel, and each pixel is constituted by either a bit pattern of 24-bit data set or a bit pattern of a 25-bit data set. The 25-bit data set bit pattern is a data array obtained by adding, through magnification correction processing, one piece of bit data to the bit pattern that corresponds to one pixel and that is originally constituted by a 24-bit data set. FIG. 14B-2 illustrates an image formed when a PWM signal is generated through the bit patterns illustrated in FIG. 14B-1 and a laser light source is driven by this PWM signal.
FIG. 14B-1 illustrates an example in which pixels that have been subjected to magnification correction are included periodically in the main scanning direction. FIG. 14B-2 indicates, as shown with a white triangle, that the width of some lines is not uniform.
The present invention is directed to providing high quality correction of image magnification that makes moire generated through interference between the periodicity of magnification correction and the periodicity of an image pattern less visually noticeable.